Purification of semi-conductor material



March 14, 1961 G. R. ANTELL ET AL 2,975,048 PURIFICATION oFSEMI-CONDUCTOR MATERIAL Filed Nov. 25, 1958 2,975,048 Patented Mar. i4,1961 ine rUnrFrcArroN or SEMI-CONDUCTOR MATERIAL George Richard Anteil,Sale, and Tudor Wyndham Roberts, Rhymney, England, assignors toMetropolitan- Vickers Electrical Company Limited, London, England, aBritish company Filed Nov. 25, 1958, Ser. No. 776,310

Claims priority, application Great Britain Nov. Z7, 1957 i' z claims.(ci. '1s-s4) This invention relates to the purification of avolatilisable metal, such as arsenic, to an extent such as tor ender itcapable of being alloyed with another metal to form a semi-conductingmaterial. The invention is also concerned with the production of thesemi-conducting material from the puried metal by an alloying processinvolving volatilisation.

Alloys of arsenic with aluminium, gallium or indium are known to possesssemi-conductivity when the alloy is in the stoichiometric proportioncorresponding with the formulae AlAs, GaAs, or lnAs respectively, or aclose approximation thereto. The presence of uncontrolled impurities inthe alloy causes large variationsin the electrical properties of thealloy. One impurity which is often associated with arsenic, and whichproduces eilects of an undesired nature, is sulphur. An other impurityis oxygen.

lt is the object of the invention to provide a purification processparticularly applicable to the removal of sulphur and oxygen fromarsenic, but applicable to `any volatilisable metal, which achieves thedesired state of purity with greater simplicity than heretofore.

According to the invention a component element of a semi-conductingmaterial capable of being volatilised and containing vaporisableimpurity is subjected to a purification process which includes the stepsof subjecting the element to oxidation in order to convert it whollyinto the oxide, removing the gaseous oxide of the vaporisable impuritysimultaneously produced, and reducing to elementary form the now pureroxide of the desired element. The reduction process may be effected byentraining the oxide, whilst being heated, in `a stream of reducing gasand carrying it into a heated reaction chamber where reduction takesplace, the entraining gas being subsequently led through a condensingchamber in which the reduced element is deposited.

If a desired alloying element is present in the condensing chamber, acontrolled introduction of the alloying element into the condensedelement may be caused to take place so that semi-conducting material isdirectly obtained.

The reduction process is preferably carried out in a succession ofinterconnected heated chambers, the direction of flow of reducing gasbeing intermittently reversed so that the entrained, partially reducedelement (arsenic) can be subjected to repetitive or prolonged action ofthe reducing gas in order that the undesired impurities are removed.

The process, as applied to the puriiication of arsenic, will `now bedescribed with reference to the accompanying drawing which representsdiagrammatically one form of apparatus for carrying out the invention.

It has been demonstrated that even spectrographically pure arsenic metalcontains traces of sulphur as an impurity and also oxygen in the form oftraces of oxide due to reaction with air. Owing to its position in theperiodic table sulphur acts as a donor impurity in the group III-group Vcompound semi-conductors which contain arsenic. To reduce the sulphurimpurity the arsenic is converted to the oxide (As203) thereby reducingthe sulphur content, the sulphur being converted to sulphur dioxide andremoved during the oxidation process. The arsenic trioxide thus formedis then reduced back to the metal in the apparatus illustrated in theaccompanying diagram.

The apparatus consists of a succession of interconnected chambers, orsections A to E through which reducing gas, e.g. hydrogen, is passedfrom an inlet F to an outlet G, the chambers being provided withsuitable heating means indicated by the interrupted lines H.

The arsenic trioxide is introduced into the chamber A which ismaintained at a temperature a few degrees below its melting point. Aslow stream of very pure hydrogen is passed up through a silica filterdisc and then through the arsenic trioxide. At 250 C. the arsenic oxidehas a low vapour pressure and so the hydrogen entrains the oxide andcarries it through chamber B which is maintained at about 250 C. andinto a reaction chamber C which is held at about 800 C. The arsenictrioxide reacts with the hydrogen in reaction chamber C and is partlyreduced to the metal. Also in the reaction chamber C any remainingsulphur is converted to hydrogen sulphide and is entrained in andcarried away with the gas stream. The mixture of arsenic and arsenicoxide vapours is carried into a condensing chamber D Which is maintainedat about -l00 C. and the vapours condense, any water vapour formed bythe reduction being swept out of the apparatus. When all the arsenictrioxide has been removed from chamber A, the direction of ow ofhydrogen is reversed and chamber D is gradually heated and distills itscontents through the chamber C where further reduction takes place, thevapours condensing in chamber B. The flow of hydrogen is again reversed,chamber B heated and chamber D cooled to about 80-100 C.

After three passes through the hot chamber C the arsenic trioxide issubstantially completely reduced. Two further passes may then be givenand the arsenic is swept into the collecting tube by heating chamber Dwith the gas stream going from left to right in the diagram. Chamber E,which acts as a collecting tube then contains pure `arsenic andhydrogen. This chamber is then evacuated, sealed oif, and removed.

If it be desired to introduce a desired alloying element into thearsenic, eg. to produce a semi-conducting Asln alloy of stoichiometricproportion, the Yamount of arsenic trioxide introduced into chamber A isof predetermined quantity. After purification of the arsenic trioxidehas been carried out, as above described, the reduced arsenic beingiinally located in chamber D, a silica boat K containing an appropriateamount of pure indium is placed in chamber E and then the arsenic sweptin from chamber D by the gas stream iiowing from left to right,following which the tube is sealed. The two elements are cornpounded bypassing a heated Zone repeatedly along the boat in both directions. Whenthe reaction is complete the hot zone is passed several times in onedirection only. This serves to sweep any unchanged indium to one end ofthe boat in accordance with the well-known zone refining technique.

The amounts of arsenic trioxide and indium employed must be carefullyrelated to one another in order to obtain the required amounts of indiumand arsenic.

What we claim is:

l. A process of preparing from arsenic containing a vaporisableimpurity, arsenic in a state of puriiication suitable to enable it toserve as a component element of a semi-conductor alloy with a metalselected from the group consisting of gallium, indium, and aluminum,

which process consists in the steps of oxidizing said arsenic and saidimpurity, heating said oxide of arsenic and entraining said oxidesVwhile heated in a stream of hydrogen, carrying said stream ofhydrogenentraining saidV oxides into a reaction chamber heated to aternperaturevsuch that solely by reaction of said hydrogen with said oxidesreduction of arsenic takes place with separation of said vaporisableimpurity therefrom, leading said traininent into said reaction chamber,condensing said distilled arsenic, and then `re-distilling the condensedarsenic with a further reversal of said stream in order again tocondense arsenic in the condensing chamber.

References Cited inthefiile of this patent UNITED STATEVSKPATENTS1,433,533 Cullen Oct. 31, 1922 V2,830,259 Jenny Apr. 8, 1958 2,858,275-Folberth Oct. 28, 1958 2,862,787 Seguin et al Dec. 211, 1958 OTHERREFERENCESV Mellor: Comprehensive Treatise on Inorganic and 15Theoretical Chemistry, v01. 1X, pp. 13 and 14, (Copy in Div. 59.) p

1. A PROCESS OF PREPARING FROM ARSENIC CONTAINING A VAPORISABLEIMPURITY, ARESNIC IN A STATE OF PURIFICATION SUITABLE TO ENABLE IT TOSERVE AS A COMPONENT ELEMENT OF A SEMI-CONDUCTOR ALLOY WITH A METALSELECTED FROM THE GROUP CONSISTING A GALLIUM, INDIUM, AND ALUMINUM,WHICH PROCESS CONSISTS IN THE STEPS OF OXIDIZING SAID ARESENIC AND SAIDIMPURITY, HEATING SAID OXIDE OF ARSENIC AND ENTRAINING SAID OXIDES WHILEHEATED IN A STREAM OF HYDROGEN, CARRYING SAID STREAM OF HYDROGENENTRAINING SAID OXIDES INTO A REACTION CHAMBER HEATED TO A TEMPERATURESUCH THAT SOLELY BY REACTION OF SAID HYDROGEN WITH